Setting parent and child windows?

I have a problem with parent and child windows. How do I set the main program to parent window and the other windows to child? I also wanna be able to change a line of text in the main (parent) window from a textbox in a child window. How would I do that?

There is a variety of ways to do this. I will show you the one I use most often.

It all boils down to variable scoping. Form2 (child) needs to be able to see Form1(parent).
There are two properties in a Form (Parent and ParentForm), however you can not rely upon them being set because of the number of ways to create a form.
Therefore, what I typically do is pass the main form instance to the child form constructor.

Once the child has this instance you can call any public method or property in the parent form. So in this example, I selected to set the modifier property of a button on the main form to public. Now Form2 can set the text of form1's button like this: